Just a small advice: I wouldn't give away what's going to happen in a story in general. A rough summary... totally fine but after reading this long summary, I pretty much know what's gonna happen and reading the actual story won't be as exciting as it'd be if you hadn't spoiled what's gonna happen 
The story idea sound alright. I will check your story out as soon as you start posting it 
Actually, that was my wiki entry text. I'm still working on the story.
I did change some parts, including, sigh, having Arnold ate one point, as he was hungry anyway, and saw the Gang of Five, decide that Ducky would be the first Flat Tooth to pay the price for the crime, he mistakenly believed, of murdering his father. His father had been really great to him, and, he always believed his father to have been a Sharptooth and been killed by Leaf Eaters (and the body destroyed) for no other reason than for being such. Anyway, he licks his lips and stares at Ducky and says "Say bye bye Leaf Eater!" (would that be too disturbing in an actual film, though perhaps not, as I thought Ozzy says something like "Say bye bye weed whacker!" when he's about to throw Littlefoot off the Great Wall.)
(You might be a Ducky fan, (personally, I love the purple guy with the red eyes. He is
so cute and he seems so under-appreciated, by his own kind and especially by herbivores. He is so sweet, and, considering the guy in the first film (not a nice guy!
) , he seems the total opposite.) so I thought I'd give you a heads up.)
Luckily, he doesn't get Ducky and instead flies into a rock, where he is discovered by the Gang of Five. He is shocked that Leaf Eaters can talk. Cera sarcastically remarks that they've been able to do that for many cold times now. Young Arnold, feeling that he may well be able to suffer, as he believes, the same fate as his father, starts pleading for them not to harm him. Cera, who doesn't yet understand the reasoning for his behavior, thinks him paranoid.
Here is more of the script: (Note, there are gaps in it)
(Arnold lands in the Great Valley. He is behind a bush. He hears Littlefoot, Cera, Ducky, Petrie, and Spike approach. He makes no indication that he can hear their conversation.)
Arnold: (Whispering) Leaf Eaters! Evil creatures! (His stomach growls.) (Focuses on Ducky) Time to make mother proud.
(The danger theme plays.)
Petrie: We sure glad to get away from your aunt.
Ducky: She sure does talk a lot. Yep yep yep she does.
Arnold: (Whispering) (Savagely) Say bye bye Leaf Eater! Time for some breakfast! (Licks lips)
(The danger theme stops playing.)
(He flies out at Ducky. However, Ducky moves and he misses and flies into a rock. The others don’t see his attack on Ducky. However, they do see him hit the rock. They run toward him.)
Littlefoot: (Concerned) Are you ok?
Arnold: (Dazed) What?
Littlefoot: (Moving over him) I asked, are you ok?
Arnold: (Shocked) You guys talk!
Cera: Yeah, we’ve done that for many cold times now.
Arnold: (Cowering) Don’t hurt me! Please! I’m just a kid!
Cera: Talk about paranoia!
Ducky: We are not going to hurt you. Oh no, no, no. We aren’t.
Cera: So, where are you from?
Arnold: (Points outside the Great Valley) Out there.
Ducky: You mean the Mysterious Beyond.
Arnold: I beg your pardon.
Littlefoot: The Mysterious Beyond. It’s what we call outside of the Great Valley.
Arnold: The Great Valley?
Ducky: It’s where we are, silly!
Petrie: You look exhausted.
Arnold’s Confession
Arnold: Ummmm, guys, there is something I must tell you. I cannot hide it from you anymore.
Littlefoot: What is it?
Arnold: (Trembling) I’m…….I’m……I’m a Sharptooth!
(The Gang of Five laughs, not believing him.)
Petrie: (Not believing him) (Laughing) Good one Arnold. How could you be a sharptooth? You no have sharp teeth.
Arnold: Reach into my mouth. I promise I won’t bite.
Petrie: (Still laughing) Ok. Me play along. (Puts hand into Arnold’s mouth. Touches a pointy tooth and pulls back) Ow, that sharp! (Realization) Wait, that one sharp!
Arnold: (Nodding miserably) Yeah. I have many like that.
(The Gang of Five gasps.)
Cera: You really ARE a sharptooth!
Arnold: Yes, I’m a Pterodactylus. A Sharptooth flyer. This is a leaf eater valley. I don’t belong here. (Turns to go)
(Petrie grabs onto him.)
Arnold: (Pleading) Please, I said I’d leave. Don’t kill me! I’m just a kid! I don’t want to die like my father!
Littlefoot: Your father?
Arnold: He was murdered by leaf eaters. They killed him because he was a Sharptooth. I never even got to find the body.
Littlefoot: We wouldn’t kill you. We have no reason to.
Arnold: (Flabbergasted) No reason to?! Look here, maybe you’re too young to get it, so let me spell it out for you: My kind eats your kind. I’ve eaten your kind. Your kind and my kind aren’t supposed to be friends.
Petrie: (Nervous) You no here to eat Petrie, are you?
Arnold: No, not what I came here for. I came here to hide from my mother.
The Gang of Five: (Minus Spike, who just trembles with the rest) Your mother?!
Arnold: Yes, she’s really bossy and doesn’t like me eating Tree Stars . I have to sneak them to get them.
Littlefoot: That’s awful!
Arnold: My mother isn’t fond of leaf eaters because of what happened to father. In fact, she said that there is only one good place for leaf eaters to go.
Cera: The Great Valley?
Arnold: Our stomachs.
The Gang of Five: (Disgusted) Ecccch!
Littlefoot: Um, Arnold, you don’t, er, believe that, do you?
Arnold: I used to think that. My father was so kind to me and I miss him greatly. (Speaking in anger) If I ever encountered the leaf eaters that did it and I got the better of them, then…..then ….. I’d enjoy every….single….bite (licks lips)!
Littlefoot: But we’re not like them.
Arnold: I can believe you. You’ve been so good to me. However, I’m not meant to be here. Now, Petrie, please let me go.
Littlefoot: Wait here. (They, minus Petrie, who still holds Arnold, turn to go.)
Arnold: What for?
(He waits nervously, trembling. Eventually, they come back.)
Arnold: Ok, what have you decided to do with me? If you’re going to kill me, please get it over with. I hate waiting. The suspense is what is killing me right now.
Littlefoot: (Gives Arnold something) Here.
Arnold: What is it? (Notices that it is a bunch of Tree Stars with bugs on them) (Confused) Huh?
Littlefoot: Enjoy!
(Arnold eats both the Tree Stars and the bugs. When he is done, he wipes his mouth with one of his wings.)
Arnold: Thank you! Wow, that was good! I’m guessing you’re not going to kill me then?
Littlefoot: Of course not! You’re not the first Sharptooth that we made friends with!
Arnold: (Taken aback) I’m not???
Littlefoot: No, we happen to have a great friend named Chomper . He’s a two-footer and a Sharptooth. He likes Stinging Buzzers
Arnold: Those are so spicy!
Petrie: That what he say too!
Arnold: Petrie, could you not tell your mother about what I told you? I’m not so sure she’d like to have a Sharptooth around her kids.
Petrie: Nonsense. Me know that she loves you.
Arnold: She loves Arnold the Leaf Eater, not Arnold the Sharptooth.
Petrie: But you no try and eat any of us. Me think that be good enough for her.
Arnold: Petrie, I have.
Petrie: (Turning pale) You have?
Arnold: (Nodding) Ducky. When I first saw you guys, I hated all Leaf Eaters on account of what had happened to my father. I was hungry and so was going to eat Ducky for revenge. I’d hoped that I could go back to mother and tell her that I’d gotten my first Leaf Eater. I knew that it would please her. However, I missed and flew into that rock.
Petrie: That explain a lot!
Cera: Hmmmmph! Some friend you are!
Littlefoot: (Upset) Cera, he was hurting! And he didn’t manage to hurt Ducky either. He’s nice now.
Arnold: No Littlefoot, she’s right, I tried to harm Ducky and deserve no mercy from you guys. My kind certainly wouldn’t have had any mercy on you had it been the other way around.
Littlefoot: I can show mercy to whomever I want, even if they or their kind wouldn’t show it back to me.
Arnold: You are certainly the kindest Leaf Eater I’ve ever met. And, quite frankly, I’ve never met any before.
Arnold Learns the Truth
Petrie: Me no get it. Chomper never able to eat Tree Stars and no Sharptooth me know, not that me know that many, can do it either.
Arnold: Yes, and mother cannot stand them. But then, why would I like them? Unless….. (Stares at Petrie) (Realization) (Excited) Petrie! Petrie! I’ve figured it out! I’m only half Sharptooth! My father was a Leaf Eater Flyer like you!
The Gang of Five (minus Spike): (Cheering) Hurray!
Arnold: (Glares at Agatha) You lied to me! You knew father was a Leaf Eater all along! These Leaf Eaters have been better to me than you ever have! You can go back alone!
(Agatha theme plays.)
Agatha: (Furious) You will go back with me or your Leaf Eater friends will end you like your father!
Arnold: You told me all this time that my father was a Sharptooth. You’ve been lying to me! You told me that he got killed by Leaf Eaters.
Agatha: Your father was getting tired of me. I suspected he would try and take you away from me. You were focusing too much on him. I had to do what I had to do.
Arnold: (Angry) What did you do to him?
Agatha: (Grinning) Well, I just….er….unrestrained my nature. (Makes a violent snapping motion with her beak) He never stood a chance.
Arnold: (Outraged) Murderer! I’m never going back with you! I’m staying here with them!
Agatha: (Grinning) (Summoning her Sharptooth Flyers with a wing) (They surround the Gang of Five.) Well Arnold, you can certainly stay here. However, my friends here a hungry for a little supper.
(Agatha theme stops playing.)
(Meanwhile)
Ducky’s Father: (Noticing the sun set) The kids should be back by now.
Mama Swimmer: Yes, I told Ducky to be back.
Mama Flyer: Where are Petrie and Arnold?
Pterano: I don’t know.
Topps: Something doesn’t seem right.
Grandpa Longneck: (Spotting the Gang of Five and Arnold on the Great Wall) Sharptooth Flyers! They’re in trouble! (They all head toward the Great Wall.)
(Meanwhile)
Agatha: (Smug) So, Arnold, you can come with me and be a Sharptooth and like it, or I can show you Sharptooth and your friends will die!
Arnold: (Resigned) Fine, I’ll go with you. But you must promise not to harm Littlefoot, Cera, Ducky, Petrie, or Spike at all.
Agatha: (Grinning) Deal! Now come on Arnold!
Littlefoot: (Yelling) Arnold no, don’t go with her!
Arnold: Littlefoot, if I don’t, you guys will die!
Ducky: We’d rather die than have you go to be miserable for the rest of your life. Oh yep, yep, yep we would.
